Conventionally, a coolant of which the main component is a melting point depression agent such as alcohols and glycols is added to the cooling water of an automobile engine to prevent the cooling water from freezing in winter. However, the alcohols and glycols not only have no antirust effect, but also are oxidized by contact with oxygen while circulating at high temperatures, and problems arise that the generated oxides promote corrosion of the metallic materials which comprise the cooling water passage.
Then, in general, a rust-prevention agent selected from a group of phosphates, borates, carbonates, sulfates, nitrates, molybdates, benzoates, silicates, benzotriazole, mercaptobenzothiazole sodium salt, tolyltriazole, and triethanolamine salts, etc. is added to the coolant to prevent the metallic corrosion while using it by mixing a predetermined amount into the cooling water.
Since a variety of metallic materials such as cast irons, steels, and copper alloys are used for the cooling system of an internal combustion engine such as automobiles, it is a requirement of the coolant to prevent corrosion regardless of the kind of metal. However, accompanying the extensive use of aluminum parts for the purpose of resource and energy conservation, it becomes obvious that a conventional coolant has insufficient anticorrosion properties against aluminum system metals.
For instance, borates have excellent anticorrosion properties against cast iron materials, but are corrosive against aluminum system metallic materials. Moreover, triethanolamine has anticorrosion properties against both iron system and aluminum system metallic materials. However, when amine exists together with a nitrite, there is a possibility that they react to generate toxic nitrosamine. Furthermore, an amine salt has a problem that the anticorrosion properties against iron decrease rapidly due to deterioration.
In addition, there has been a movement to discuss the adoption of parts made of magnesium alloys for the cooling system for the purpose of both resource and energy conservation. However, magnesium has the lowest standard electrode potential and low corrosion resistance in practical metallic materials, so that it is necessary to consider the corrosion resistance in a practical use. Improvement in the corrosion resistance is considered to be achieved by the cooling water used for the cooling system as well as by improving the corrosion resistance of the magnesium alloy itself. However, a conventional coolant additive cannot prevent magnesium from corrosion.
As a coolant which can prevent magnesium from corrosion, WO00/22189 discloses corrosion inhibitor compounds including a) 0.1 to 15 wt % of one or more inhibitors selected from a group of alkylbenzoic acids, C5 to C15 monobasic acids and C5 to C15 dibasic acids, and salts thereof, b) 0.005 to 5 wt % of fluorides and/or fluorocarboxylic acids and salts thereof.
However, it has been a problem that fluorides have excellent anticorrosion properties against magnesium but have adverse effects on aluminum alloys.